Startpos editing cannot be done in RPFM as usual we are instead forced to use the Assembly Kit, usually called Dave. Many things can be changed mid-campaign, like building costs or recruitable units, but startpos is essentially baked into the campaign from the start - it's how the game knows what settlements exist and where, where to initially put lords, what units they have, who is playable, etc.
It's typically called startpos modding because it uses the startpos.esf file, which dictates the initial setup of a new campaign. With that all said, let's discuss what faction creation is. I'm not trying to discourage you from trying it - it's not that HARD - but it can be time consuming to get the hang of and even for startpos experts it can be a pain. If anything is wrong, Dave and Bob may or may not let you know at all, compiling may or may not fail without any useful reason, and even if it does succeed the game may crash during loading for no apparent reason. This isn't like normal modding where you can easily reverse engineer other mods to figure out how they work, and most errors have a clear or clear-ish cause. First of all, I would like to be brutally blunt here: startpos modding sucks.
